1. Field of the Invention
The present invention relates to spread spectrum receivers of the type used in military communications receivers. More particularly, the present invention is concerned with eliminating the harmful effects of burst jamming signals that are employed by unfriendly forces seeking to disrupt or interfere with communications signals being received at the spread spectrum receiver, and is equally effective in eliminating the effects of unintentional and/or friendly interference signals such as radar signals.
2. Description of the Prior Art
Our U.S. Pat. No. 4,435,822 shows and broadly describes a coherent spread spectrum receiver which employs automatic gain control (AGC) elements. The receiver shown in this patent without modification could be jammed by unfriendly burst signals. Prior art attempts to mitigate the effects of burst jamming have consisted of using (1) broadband limiters and (2) a burst detector coupled with a muting circuit. The purpose of the latter circuit is to shut-off the receiver during the burst period. Heretofore, limiters have had the desirable fast reaction time but are inherently single threshold devices which makes them ineffective for bursts that have markably different amplitudes. Another disadvantage of prior art limiters is that for other types of jammers, such as CW, FM, AM or noise, when limiting occurs, inband intermodulation products are generated that seriously deteriorate the desired signal.
The burst detector/muting circuit approach also is inadequate for coping with a burst jammer. This method again is essentially a fixed threshold device that limits its effectiveness against bursts of diverse amplitudes; in other words, there is a proper threshold for a given burst amplitude but this threshold will be wrong for other bursts with different amplitudes. In general, any threshold chosen to minimize the burst energy will be sufficiently low that signals, other than burst signals, will cause the circuit to false alarm at an excessive rate and thus lend the signal unusable. If the threshold is set high enough to avoid false alarm problems, then energy pulses which do not exceed the threshold will cause severe signal suppression along with an increased probability that many of the pulses will not be detected.
It would be extremely desirable to provide a burst suppression circuit that is fast, provides automatic threshold levels, will not false alarm against other types of jammers, is simple to implement, and may be retrofitted with a minor modification to existing receivers.